The present invention relates to a method for improving the steering response in motor vehicles with superimposed steering system at the front axle, wherein the superimposed steering system includes a superimposed transmission and wherein, controlled by a control device, a superimposed steering angle is imposed by means of the superimposition gearing on the steering wheel angle resulting from the actuation of the steering wheel, which superimposed steering angle increases or decreases the steering angle depending on at least the measured speed of the motor vehicle.
The following discussion of related art is provided to assist the reader in understanding the advantages of the invention, and is not to be construed as an admission that this related art is prior art to this invention.
The recent past has seen numerous proposals to use active front axle steering systems in motor vehicles. These enable a steering intervention on the front axle independent of the steering wheel angle. In standard motor vehicles available to date so-called superimposition steering systems are installed because these ensure in the case of failure of the electronic system the steerability via the mechanical connection between steering wheel and steering gear. In this special type of active steering an angle is superimposed on, i.e., imposed on a steering wheel angle that is set by a steering wheel. The steering angle at the output of the steering gear then no longer corresponds to the angle set by the steering wheel. This measure on one hand enables a stepless adjustment of the steering gear so that for example a direct transmission ratio can be set for maneuvering at slow speeds, which requires fewer turns of the steering wheel during parking, while during driving on the highway an indirect transmission ratio can be set which renders control over the vehicle more sensitive. On the other hand a superimposition also enables fast driving dynamics interventions. This means that during the drive, to the most degree independent of steering interventions by the driver, the wheel angle can be changed which enables a fast responding regulation of the driving dynamics.
In superimposition steering systems it is known to use measuring values, vehicle parameters and vehicle specific variables for determining the steering ratio or the transmission ratio that actually has to be adjusted. This includes in particular the actual vehicle speed and the actual steering wheel angle. Using these variables the steering ratio is determined with a model, a characteristic curve or a characteristic field, from which steering ratio in turn a superimposition angle for the superimposition gearing can be determined. The superimposition angle is applied by controlling an actuator for example an electric motor, which sets the superimposition angle at the superimposition gearing.
Known are superimposition steering systems of the type described above for example from DE 197 51 137 A1. This reference relates to a steering system for a motor vehicle with a steering drive train, in which at least one steerable wheel, an actuating drive and a superimposition gearing and the steering wheel which can be actuated by the driver of the vehicle and also mechanical deflection means are arranged.
In DE 10 2006 008 156 A1 a method for determining the steering transmission ratio of a vehicle is described, wherein during a stable drive of the vehicle the steering transmission ratio is determined from detected measurement values by taking vehicle parameters into account.
A common feature of the state of the art is that due to the increasingly indirect steering transmission, the steering response of the motor vehicle starting from the neutral straight ahead drive deteriorates in particular in the mid to upper speed range insofar as a turning of the steering wheel only results in a small steering reaction at the steered wheels. The steering response thus becomes increasingly indirect with increasing vehicle speed and causes a “fuzzy” steering experience in particular in the range about the neutral center position of the steering wheel.